PCA-STREAM

The challenges of timber construction in addressing the climate emergency

The construction sector accounts for one third of France’s CO₂ emissions. This environmental footprint can be reduced by using wood, but wood only stores carbon temporarily. The challenge of sustainable construction therefore lies in a comprehensive approach that includes forest management and the durability of buildings. No ‘woodwashing’ here, but some thoughts on mixed construction!

Visible timber structures in architecture promote well-being. The hygroscopic properties of wood help regulate humidity and increase the levels of perceived comfort. On top of that, various wood types have different scents contribute to improving focus, sleep, and calm.(Sakuragawa, 2005)

The construction sector represents one-third of France’s CO2 emissions, and, of that, the production of materials contributes 16%.(Carbone 4, 2019) Unlike steel and concrete, wood has low embodied energy. Its capacity for self-renewal without any fossil fuel input as well as its ability to store carbon makes it a promising way to fight global warming. France’s National Low-Carbon Strategy aims to increase the production of wood sold in France by 30%(AGRESTE, 2017) in ten years (between 2016 and 2026),(Ministère de la transition écologique et solidaire, 2018) offering a new horizon to the construction sector.

While wood can be used to achieve lightweight, prefabricated and precise constructions, it isn’t adapted to all situations. Furthermore, it is part of a larger cycle of resource management, production, processing, marketing, and so on, that may offset its lower ecological footprint.

Where should wood be used? To what extent? What systemic vision should we adopt to avoid falling prey to ‘woodwashing’ and create a sustainable wood industry? Here are a few insights that weigh in favor of mixed construction.

Carbon sequestration in wood is substantial but short-lived

As trees grow, they capture around 1.5 kg of CO₂ for every kilogram of wood produced.(Peuportier, 2021) The carbon that is absorbed in the atmosphere by plants through photosynthesis is called biogenic carbon. Any such carbon that is stored in the trunks of the trees remains in the transformed wooden building materials. One cubic meter of CLTCross Laminated Timber, bois lamellé croisé, un matériau de construction à base de bois qui se présente sous la forme d'un panneau multi-couche  contains the equivalent of 700 kg of CO₂ captured in the atmosphere(Carbone 4, 2013) and has a negative carbon footprint of -520 kg of CO₂ (given that wood processing emits 180 kg of CO₂ per cubic meter of wood). In comparison, producing one cubic meter of concrete generates 210 kg of CO₂.(CEMII, C25/30)  (Ifpeb, carbone 4, 2020)

But the sequestration of biogenic carbon doesn’t last forever. When the wood decomposes or is burned, the carbon is eventually released back into the atmosphere. Building with wood helps even out emissions over time, but in no way does it lead to neutralizing them. These emanations, though delayed to the end of life of products, will ultimately have an impact on climate change. The onus is on the computational tools and regulations to make sure that these emissions are accounted for in keeping with long-term climate goals.(Peuportier, 2021)

 

The challenge: the longevity of architecture

The service life of wooden building components is estimated to be around 50 to 100 years. Nevertheless, many historical buildings boast timber structures that attest to the fact that wood can last for centuries. It is when buildings become obsolete, rather than when wood fails, that wooden buildings tend to be dismantled. Extending the service life of buildings and products by promoting regular maintenance and rehabilitation operations rather than demolition-reconstruction and reusing materials will help delay the CO₂ emissions that are generated at product end-of-life.

Combining materials: towards more construction intelligence

In order to extend the longevity of buildings, it is crucial that they be made more flexible in order to accommodate possible shifts in use. Wood doesn’t always meet construction requirements, however. Unlike concrete, it does not isolate from capillary moisture and is therefore not particularly well adapted to basement environments. Among its other notable drawbacks are its acoustics and fire safety. For example, in a commercial building, in order to accommodate the public and various programs on the ground floor, including catering areas, a concrete base will provide more flexibility, and therefore more durability over time.

Hybrid construction systems, combining timber with concrete or steel, mitigate the shortcomings of each kind of material, while also shortening construction times. With a hybrid structure made up of 60% wood (posts, beams, floors) and 40% concrete (core, stairs), a 1,200 m2 floor can for example be built in 9 days.

 

The need for sustainable forest management

Beyond architectural prescription, a national discussion must take place. Though the storage of biogenic carbon is conditional on the lifespan of timber construction elements, forest management is also key.

In order to match the growth in timber construction, logging must increase in France, which carries the risk of deteriorating forest resources and forest management quality.  

When a tree is cut down to manufacture a construction product, it ceases capturing carbon. We must therefore ensure that a new tree is then replanted to ensure that forests continue being reliable carbon sinks. Replacing trees isn’t enough. Continuous cover regimes are preferable over clear-cutting as the felling of all trees in a plot disturbs the soil, inducing CO₂ emissions that may be greater than the carbon absorbed by the growth of new trees during the first years of the forest’ life.(Gaëtan du Bus de Warnaffe, 2020) Clear cuts should therefore be banned and the harvesting of trees must be consistent with the aim of ensuring positive net absorbed CO₂.

Management policies will determine the extent of the role played by French forests in the fight against global warming, as well as our ability to integrate industrial production with care for ecosystems.(Ministère de l'agriculture et de l'alimentation, 2019)

Building up the local processing industry

With a forest cover of 30%, France has the 4th largest forest area in Europe.(Office National des Forêts, 2021) Yet less than 50% of the wood that grows in the country each year is harvested. This is due to the forest ownership fragmentation and dominant private forest ownership, as well as the low proportion of conifer forests that are of interest to the industry.(Fédération Nationale du bois, 2021)

The country is struggling to position itself in the sectors that create value. Importing a product processed abroad remains cheaper than producing it locally,(BOTREL, 2015) which is why France, which is Europe’s third largest producer of round logs by value, exports the raw material and then imports value-added products such as panels, furniture, paper, and cardboard. In 2020, this sector of the economy generated a foreign trade deficit of 7 billion euros.(AGRESTE, 2021) However, the development of the wood processing industry in France would reduce product imports, allow local wood processing, and help achieve a better trade and carbon balance.

Unlocking the potential of wooden construction

Wood-based construction has an important role to play in meeting the challenges of climate change and can help reduce emissions from the construction sector. But these carbon benefits can only occur through a systemic approach including improving the longevity of buildings, ensuring sustainable forest management, developing the French wood processing industry, and broadening the possibilities for the use of wooden construction products in government regulations.

 

Laélia Vaulot, Low Carbon Manager at PCA-STREAM